1. Field of the Invention
The present invention relates to a spin immunoassay apparatus for monitoring the concentration of drug used for therapeutic purpose in physiological fluids.
2. Prior Art
There have heretofore been used immunoassay apparatus (known as radioimmunoassay apparatus) for enabling the operator to observe antigen-antibody reactions with a radioisotope used as the labelling agent in monitoring the concentration in physiological fluids and the amount of excretion of a variety of drugs used for therapeutic purpose. Since the prior apparatus relies on the radioisotope, the operator and other persons involved in the handling of the apparatus are likely to be exposed to radiation, and the apparatus requires expensive facilities to dispose of the used radioisotope which would otherwise cause environmental contamination if left abandoned. The radioisotope is unstable as it has a certain half-life. The known apparatus are also disadvantageous in that they take a lengthy period of time, a few hours to twenty-four hours, for measurement which involves a preparation stage up to a post treatment stage, and hence are unsuitable for use with emergency tests.
The foregoing difficulties could be avoided by using a spin immunoassay apparatus which relies on stable free radicals as a labelling agent in enabling the operator to observe antigen-antibody reaction. More specifically, a sample (human urine, for example) is put into a test tube and a reagent made of a spin labelled antigen is added to the sample. The mixture is sufficiently blended, incubated for a certain interval of time in a bath of water at a temperature of 37.degree. C., and transferred into a sample cell. The sample cell is inserted into a cavity resonator of an electron spin resonance (ESR) device so as to produce an ESR spectrum of the spin labelling agent. The difference (peak-to-peak value) between maximum and minimum differential values of the spectrum can be regarded as the concentration of free lavelled antigen released from antibody. With this process, however, sample cells tend to be located at varying positions in the cavity resonator causing fluctuation of the quality factor of the cavity resonator. Therefore, the peak-to-peak values greatly vary according to the position of the cell in the cavity resonator.
A process proposed to eliminate the above problems would be to place a calibration sample (for instance, a substance containing Mn.sup.2+) in the cavity resonator which remains left in the cavity resonator, measure the ESR spectrum of the calibration sample and a sample to be tested at the same time, and calibrate the ESR spectrum of the sample tested with the ESR spectrum of the calibration sample. The proposed process however suffers from the following defects: first, although a peak-to-peak value of the sample being tested can be obtained by sweeping the intensity of the magnetic field within just the range of resonant signal, a resonant signal for calibration from the calibration sample should be measured at the same time, resulting in an increased area to be swept and a prolonged period of time required for measurement; secondly, when a signal which is much less (for example, 1/100) intensive than a calibration signal which is automatically calibrated by a data processor, digital noises reduce accuracy of measurement even if the calibration signal is adjusted to have its maximum value scaled by full bits of an analog to digital (A/D) converter, since the number of bits assigned to the signal being tested is 1/100 of the number of the full bits. Where the A/D converter has a resolving power of 10 bits and an accuracy of 1/2, the least significant bit (LSB), an error of .+-.5% occurs at this stage; thirdly the proposed process is tedious and time-consuming.